High energy product, high coercivity permanent magnet compositions comprising, for example, iron, neodymium and/or praseodymium, and boron and methods of making them are disclosed in U.S. Ser. No. 414,926 filed Sept. 3, 1982, Ser. No. 508,266 filed June 24, 1983 and U.S. Ser. No. 544,728 filed Oct. 26, 1983, all by John J. Croat and assigned to the assignee of this application. An illustrative composition, expressed in atomic proportions, is Nd.sub.0.13 (Fe.sub.0.95 B.sub.0.05).sub.0.87. It is a composition containing a specific stable intermetallic phase and that possesses high coercivity when formed as fine crystallites about 20 to 40 nanometers in largest dimension.
Melts of suitable iron-light rare earth metal-boron compositions can be very rapidly quenched, such as by melt spinning, to produce a solid material, e.g., a thin ribbon. When the rate of cooling has been controlled to produce a suitable fine crystalline microstructure (20 nm to 400 nm), the material has excellent permanent magnet properties. On the other hand, faster cooling (overquenching) produces a material with smaller crystallites and lower coercivity. However, as disclosed, such overquenched material can be annealed to form the suitable crystal size with the associated high coercivity and high energy product.
An interesting and useful property of this neodymium-iron-boron composition (for example) is that it is substantially magnetically isotropic. A fine grain, melt spun ribbon can be broken up into flat particles. The particles can be pressed in a die at room temperature to form a unitary body of about 85% of the material's density. Bonding agents can be employed before or after the compaction. The making of such bonded magnets is disclosed in U.S. Ser. No. 492,629 filed May 9, 1983 by Robert W. Lee and John J. Croat and assigned to the assignee hereof. It was surprising to find that such bonded magnets displayed no preferred magnetic direction. Values of intrinsic coercivity or maximum energy product were not dependent upon the direction of the applied magnetic field. There was no advantage in grinding the ribbon to very fine particles and magnetically aligning the particles before compaction.
Such magnetically isotropic materials are very useful because they can be easily pressed (without magnetic alignment) into bonded shapes. The shapes can be magnetized in the most convenient direction.
The iron, neodymium, boron type compositions have also been processed by hot pressing and hot working so that at least a portion of the grains or crystallites was physically aligned producing at least partial magnetic alignment. As disclosed in U.S. Ser. No. 520,170 filed Aug. 4, 1983 by Robert W. Lee, such hot worked materials had a preferred direction of magnetization. In one form of the practice disclosed in that application, a molten material containing, in terms of atomic proportions, Nd.sub.0.13 (Fe.sub.0.95 B.sub.0.05).sub.0.87 is cooled extremely rapidly, as by melt spinning, to form a thin ribbon of solid material that did not have permanent magnet properties. The material was amorphous in microstructure. The ribbon was broken into particles of convenient size for a hot working operation. The particles were heated under argon to about 700.degree. C. or higher in a die and pressed with punches in the die under pressure of at least 10,000 psi. Such hot working, hereafter termed hot pressing, consolidated the particles into a fully dense body.
If the hot working is stopped at the point at which the material is consolidated to full density, the result is a slightly magnetically aligned magnet with the easy magnetization direction parallel to the press direction. The demagnetization curve (room temperature, second quadrant, 4.pi.M versus H plot) of such a densified magnet is like that of curve A in FIG. 1.
When the fully dense compact is repressed under like conditions of elevated temperature and pressure in a larger die cavity, the compact undergoes considerable plastic strain in the plane perpendicular to the press direction. This second stage of hot working is termed die upsetting, and it produces substantial magnetic alignment with the easy direction of magnetization transverse to the plastic strain direction. The demagnetization curve for such a die upset body is like that of curve B in FIG. 1. Examination of the demagnetization curves of FIG. 1 shows that the hot pressed magnet (curve A) and the die upset maqnet (curve B) have substantially different degrees of magnetic alignment. The hot pressed magnet has relatively higher coercivity and lower remanence in the press direction than the die upset magnet. The die upset magnet has a higher maximum energy product, but it is easier to demagnetize than the hot pressed body of the same composition. There are applications for magnets in which it is desirable to incorporate both characteristics in a unitary magnet body.
It is an object of the present invention to provide a method of hot working a permanent magnet body to produce at least two regions spaced along a surface dimension of the body and having different desired magnetic alignments. Generally speaking, one of the regions will have higher apparent coercivity but lower remanence than the other region. We particularly contemplate application of the method to rapidly cooled compositions comprising iron, neodymium and/or praseodymium and boron.
It is another object of our invention to provide a unitary magnetic structure that has been selectively hot worked such that it contains separate regions of differing magnetic alignment. An example of such a magnetic structure is an arcuate magnet for a permanent magnet motor. It is contemplated that one or both of the circumferential edges of the arc would be worked so as to have relatively high coercivity and the central portion of the arc would have a relatively higher remanence. Again, we especially contemplate that the magnet would be of the above described iron-light rare earth metal boron compositions.